C#： winform 不能修改app.config

查了很多文章都介绍用下面的方法修改app.config中的值：
Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
config.AppSettings.Settings["DB_UID"].Value = "demo";
config.Save(ConfigurationSaveMode.Full);
事实证明这种做法是不可行的。虽然程序运行时的值已经被修改。但是打开app.config查看，会看到值仍然没有被改动。如果重启程序，就会使用以前的旧值。用这种方法只可以读取值。不能写入或者修改值。

正确的做法是把app.config当成一般的xml文件，进行修改。
XmlDocument xDoc = new XmlDocument();
xDoc.Load(System.Windows.Forms.Application.ExecutablePath + ".config");
XmlNode xNode;
XmlElement xElemUID;
xNode = xDoc.SelectSingleNode("//appSettings");
xElemUID = (XmlElement)xNode.SelectSingleNode("//add[@key='DB_UID']");
xElemUID.SetAttribute("value", Encrypt.Create().encryptData(this.dbUserTXT.Text));
xDoc.Save(System.Windows.Forms.Application.ExecutablePath + ".config");

       private void UpdateConfig(string Xvalue)
        {
            XmlDocument doc = new XmlDocument();
            doc.Load(AppConfig());
            XmlNode node = doc.SelectSingleNode(@"//appSettings");

            XmlElement ele = (XmlElement)node.SelectSingleNode(@"//add[@key='BIRelease']");
            ele.SetAttribute("value", Application.StartupPath);
            doc.Save(AppConfig());

           
        }

        public  string AppConfig()
        {
            int intPos = Application.StartupPath.Trim().IndexOf("bin")-1;

            string strDirectoryPath = System.IO.Path.Combine(Application.StartupPath.Substring(0,intPos), "App.config");

            return strDirectoryPath;
        }

        public  string GetValue(string appKey)
        {
            XmlDocument xDoc = new XmlDocument();
            try
            {
                xDoc.Load(AppConfig());
                XmlNode xNode;
                XmlElement xElem;
                xNode = xDoc.SelectSingleNode("//appSettings");　　　　//补充，需要在你的app.config 文件中增加一下，<appSetting> </appSetting>
                xElem = (XmlElement)xNode.SelectSingleNode("//add[@key='" + appKey + "']");
                if (xElem != null)
                    return xElem.GetAttribute("value");
                else
                    return "";
            }
            catch (Exception)
            {
                return "";
            }
        }